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Differential Choice?? HELP
sammini7 - 9/12/17 at 10:26 PM

Hi,

I have a Honda K20A engine, RX8 gearbox and Sierra Cosworth Hubs, i'm going to running about about 240BHP. What differential should I be looking at guys?

Kind Regards

shindha - 10/12/17 at 09:19 AM

I think you need to provide more information like do you want a car that accelerates fast or has a high top speed what ratios the gearbox is running, then someone with a lot more knowledge than me on here will be able to help guidance.

CosKev3 - 10/12/17 at 10:16 AM

I know people have said the first gear on the RX8 box is a waste of time with the majority of the diffs used in kit car builds,they pull away in second and use it as a 5 speed box!

snapper - 10/12/17 at 11:30 AM

Your running Cosworth rear hubs, I would probably want to run Cosworth driveshafts and therefor a Cosworth 7/12” diff would be ideal, that should have a Ford viscous LSD in it and will probably have a 3.64 diff ratio

mcerd1 - 11/12/17 at 09:26 AM

quote:
Originally posted by snapper
Your running Cosworth rear hubs, I would probably want to run Cosworth driveshafts and therefor a Cosworth 7/12” diff would be ideal, that should have a Ford viscous LSD in it and will probably have a 3.64 diff ratio

^^ depends if its cossie 2wd hubs or 4x4 hubs

as you say cossie 2wd's all used the 108mm CV joints and only had 7.5" 3.64 diffs

but the cossie 4x4's and granada cossies used 100mm CV joints (the same joints as the normal sierra, but with different hub carriers for bigger rear discs)

CosKev3 - 11/12/17 at 03:12 PM

3.64 will make it very low geared,first gear will be basically a waste of time!

PorkChop - 12/12/17 at 12:03 AM

quote:
Originally posted by CosKev3
3.64 will make it very low geared,first gear will be basically a waste of time!


Diff ratio is not quite the reason - standard RX-8 diffs are a bit shorter than 3.64:1 IIRC.

However, a small wheel and tyre combo (which has not been mentioned at all, the RX-8 would've come with something like 18" wheels) would make an RX8 box and 3.64:1 diff setup short geared.

For the OP, you need to know

Rolling circumference i.e. wheel and tyre size
The ratios for whichever RX-8 box you're using (the 5 and 6 speeds are different IIRC)

Then you can start to get an idea of what sort of diff ratio you need overall. If you can use Excel, you can probably modify the gearing calculator available in the BEC section quite easily to get an idea of where you'd end up.

CosKev3 - 12/12/17 at 02:30 PM

quote:
Originally posted by PorkChop
quote:
Originally posted by CosKev3
3.64 will make it very low geared,first gear will be basically a waste of time!


Diff ratio is not quite the reason - standard RX-8 diffs are a bit shorter than 3.64:1 IIRC.

However, a small wheel and tyre combo (which has not been mentioned at all, the RX-8 would've come with something like 18" wheels) .


99.9% sure it won't be running 18 inch wheels will it?

So yeah,thanks for the pointless info

theduck - 12/12/17 at 10:50 PM

Not only does the Rx8 have 18s, it’s revs to 9k rpm!m


Oh and you either want a 3.14 or accept that first will be useless and go with a 3.6

[Edited on 12/12/17 by theduck]

PorkChop - 13/12/17 at 12:03 AM

The question here is "does the change in FD ratio alter the gearing sufficiently to allow a small enough wheel / tyre combo to be used?"

I suggest you do the calculations and you will see how pointless the info I gave is.

Feel free to disprove the numbers below; I was hungry and my tea was nearly ready...

The RX-8 can have a 9000 rpm redline. If it's a 6 speed 231. A 5 speed 192 has a much lower redline of about 7500 rpm (IIRC).

For the sake of it, let's say the redline is 7200 rpm in all scenarios. I've used Chris Gamblin's Excel calculator from the BEC section, plugged the different numbers in to do these calculations, and using the MX5 data, double checked the Excel calculator with the gearing calculator on Flyin Miata's website.

So...

Assuming the RX-8 has 225/45/18 tyres, the RX-8 gearbox and FD ratios are as per www.rx8club.com/rx-8-discussion-3/final-gearing-ratios-more-come-promise-1660 you have the following vehicle speeds at 7200 rpm in 1st gear;

RX8 192 5 speed - 36 mph
RX8 231 6 speed - 33 mph (bear in mind that you are still some way away from the actual redline of 9000 rpm, so the redline speed in 1st gear is higher in reality)

If we take each RX8 gearbox and mate it with a 3.62:1 FD and a not unusual 195/50/15 wheel / tyre combo;

RX8 192 5 speed - 38 mph
RX8 231 6 speed - 35 mph

To put this into context, a mk1 MX5 on 195/50/15s will be doing 38 mph at 7200 rpm in 1st gear. My mk3 MX5, on 16" wheels, would be just shy of 40 mph. Trust me, that car is not short geared!

If the numbers above are anywhere near right, do they sound that short geared to you? They don't to me.

[Edited on 13/12/17 by PorkChop]

[Edited on 13/12/17 by PorkChop]

froggy - 13/12/17 at 12:30 AM

I found a site to put all the numbers in and gives you rpm v speed for each gear . I run 245/45/16 rear with a 3.64 diff and 5sp with fifth 0.73 . 1st is short but useable but the speed spread between 3-5th is bang on for fast road and track . Shift points for 50-100mph and cruise rpm at 70 is more useful info . I’ve a feeling the Westfields with a duratec use a Mazda 5 or 6sp from the mk3 mx5 that might have the same ratios

sammini7 - 24/12/17 at 11:20 AM

Thanks guys for all you help on this one !